This invention relates to a C-V(capacitance-to-voltage) conversion apparatus which converts a capacitance change of a capacitive sensor into a d.c. voltage signal, and more particularly to a C-V conversion apparatus for a sensor which is of great use in detecting changes of physical parameters such as pressure, distance and angle.
Capacitive sensors mainly composed of parallel plate electrodes printed on parallel diaphragms (made of crystal or alumina) are well-known. For example, a pressure-sensing capacitor has been introduced in U.S. Pat. No. 3,858,097 as a typical capacitive sensor. This type of capacitive sensor is mainly composed of two parallel plate electrodes facing each other and two parallel diaphragms spanning a gap therebetween. The peripheries of the two parallel diaphragms are fixed to each other. The two parallel plate electrodes are printed one each on the two diaphragms, respectively. Usually, one of the diaphragms is used as a fixed diaphragm so that the plate electrode printed on it is used as a fixed electrode, and the other diaphragm is used as a movable diaphragm so that the plate electrode printed on it is used as a movable diaphragm. In other words, a capacitor is formed by the fixed plate electrode and the movable plate electrode. The movable diaphragm is supposed to move according to changes of the physical parameter so that the gap between the two plate electrodes changes. Accordingly, the capacitance between the two electrodes changes corresponding to changes of the physical parameter.
For obtaining a d.c. voltage signal corresponding to this capacitance change, some types of C-V conversion apparatus have already been introduced. The above said U.S. Pat. No. 3,858,097 exhibits a C-V conversion apparatus composed of an operational amplifier and a transformer. C-V conversion apparatuses using diode-quad bridges are exhibited in U.S. Pat. No. 3,869,676 and Japanese published unexamined patent (Kokai) No. 54-115175.
However, these conventional C-V conversion apparatuses have the following problems:
(i) They have complex circuit configurations for compensating for the non-linearity of the C-V conversion characteristics.
(ii) It is necessary for them to be provided with elements having the completely same electrical and physical characteristics, such as completely the same four diodes.
(iii) Circuit adjustments for the initial balanced states of the apparatuses are difficult.
(iv) The capacitive sensor is treated as an ideal parallel plate capacitor. In other words, such effects are not practically considered such as non-uniform deviation of the movable diaphragm between the neighborhood of the center and the neighborhood of the periphery. Because of this non-uniformity of deviation, the capacitive sensor actually is not an ideal parallel plate capacitance. This must be considered when highly accurate detection of the physical parameter is required.